1. Field of the Invention
The present invention relates to an amplitude deviation correction circuit, and more particularly to an amplitude deviation correction circuit which corrects an amplitude deviation between two baseband signals of an I signal and a Q signal after orthogonal demodulation in a receiver for an orthogonal phase modulation signal which employs a direct conversion system.
2. Description of the Related Art
Conventionally, orthogonal phase modulation is used for a digital portable telephone receiver (PDC (Personal Digital Cellular system), a PHS (Personal Handy phone System) receiver and a CDMA (Code Division Multiple Access) receiver. In the receivers mentioned, a heterodyne system is used most popularly wherein a received radio frequency signal is converted into an intermediate frequency signal and then two baseband signals are produced by an orthogonal demodulator, whereafter decoding is performed for the baseband signals by a baseband processing section in the following stage.
FIG. 5 shows an example of a conventional receiver of the heterodyne system. Referring to FIG. 5, the conventional receiver of the heterodyne system includes an antenna 1, a radio frequency band-pass filter (RF BPF) 2, a low noise amplifier (LNA) 3, an image filter (IM BPF) 41, a first mixer (MIXER) 42, an IF (intermediate frequency) filter (IF BPF) 43, a first local oscillator 25, an AGC (automatic gain control) circuit 7c, an orthogonal demodulator 4, and a pair of baseband filters (BB BPF) 5 and 6.
The AGC circuit 7c includes variable gain amplifiers (VGA) 8 to 11 and a gain controlling voltage generation circuit 16. The orthogonal demodulator 4 includes an amplifier 31, a pair of double-balanced mixers 32 and 33, and a 90° phase branching unit (0/90) 34.
In operation, a radio frequency signal received by the antenna 1 is band-limited by the radio frequency band-pass filter 2 so that a signal of a reception band is extracted. The band-limited signal is amplified by the low noise amplifier 3 and then passes through the image filter 41 which removes an image frequency of the first mixer 42. An output of the image filter 41 is mixed with a first local signal produced by the first local oscillator 25 by the first mixer 42 so that it is converted into a signal of an intermediate frequency. The intermediate frequency signal (IF signal) is band-limited by the IF filter 43 so that adjacent channel components are suppressed.
An output of the IF filter 43 is amplified by the AGC circuit 7c so that the average amplitude thereof may be fixed. Description of a circuit and an algorithm for controlling the gain of the AGC circuit 7c is omitted herein because they do not have a direct relationship to the present invention. The dynamic range of the AGC circuit 7c reaches several tens dB (approximately 80 dB with a CDMA receiver). The IF signal having the controlled amplitude is orthogonally demodulated into two baseband signals of an I signal and a Q signal with a signal from a second local oscillator 44 by the orthogonal demodulator 4. The I signal and the Q signal are outputted as signals 23 and 24 after they are band-limited by the baseband filters 5 and 6, respectively.
A receiver of the heterodyne system used most popularly at present has such a construction and operates in such a manner as described above. The receiver of the heterodyne system having the construction described above has a problem in that it is difficult to use a large scale integrated circuit to form the receiver and thus difficult to miniaturize the circuit and reduce the number of parts in the future. For example, the image filter 41 is required in the preceding stage to the first mixer 42. Also the IF filter 43 is required for processing in the intermediate frequency band. In the present stage, the elements mentioned are each formed from a passive element such as a SAW (surface acoustic wave) element or a dielectric filter and cannot be incorporated readily into an IC. Further, a second local signal is required by the orthogonal demodulator 4, and the second local oscillator 44 is required to generate the second local signal.
In the future, it is estimated to be requested to form also a high frequency circuit as a large scale integrated circuit and significantly reduce the scale of an apparatus, and therefore, the currently employed construction must be reviewed. One of possible solutions resides in a direct conversion system. A construction of a receiver of the direct conversion system is shown in FIG. 6.
Referring to FIG. 6, the receiver of the direct conversion system includes an antenna 1, a radio frequency band-pass filter 2, a low noise amplifier 3, an orthogonal demodulator 4, a pair of baseband filters 5 and 6, and an AGC circuit 7a. The orthogonal demodulator 4 includes an amplifier 31, a pair of double-balanced mixers 32 and 33, and a 90° phase branching unit 34. The AGC circuit 7a includes variable gain amplifiers 8 to 15 and a gain controlling voltage generation circuit 16.
In operation, a radio frequency signal received by the antenna 1 is band-limited by the radio frequency band-pass filter 2 so that a signal of a reception band is extracted. The band-limited signal is amplified by the low noise amplifier 3 and inputted as it is to the orthogonal demodulator 4. While the orthogonal demodulator 4 is driven with a local signal produced by the local oscillator 25, the local signal has a frequency equal to the central frequency of the radio frequency signal to be received. Baseband signals are produced directly from the radio frequency signal by the orthogonal demodulator 4.
According to the construction of the receiver of the direct conversion system described above, since no image frequency is involved, the image filter 41 is unnecessary. The baseband signals are two I and Q signals and are amplified by the AGC circuit 7a so that the average amplitudes thereof may be fixed after they are band-limited by the baseband filters 5 and 6, respectively. Since a circuit and an algorithm for controlling the gains of the baseband filters 5 and 6 do not have a direct relationship to the present invention, description of them is omitted herein. The dynamic range of the AGC circuit 7a reaches several tens dB (approximately 80 dB with the CDMA). Outputs of the AGC circuit 7a are outputted as signals 23 and 24 to the following stage.
In the receiver of the direct conversion system, channel filters for suppressing adjacent channels are implemented not from SAW filters of the IF band but from the baseband filters 5 and 6. Since the elements can be implemented with a circuit which employs an active element, they are suitable for formation into an IC. Further, since a radio frequency signal is converted directly into a baseband signal, the second local oscillator 44 is unnecessary. Furthermore, also the image filter 41 is unnecessary as described hereinabove. Therefore, there is the possibility that all elements of the reception circuit from the low noise amplifier to the baseband output can be formed into a single chip. This contributes very much to miniaturization and reduction in number of parts of a portable telephone set.
However, the receiver of the direct conversion system has the following problem. While, according to the conventional construction of the receiver of the heterodyne system, an AGC circuit may be provided at only one place for an IF signal, according to the receiver of the direct conversion system, it is necessary to provide AGC circuits separately for an I signal and a Q signal of the baseband.
As shown in FIG. 6, on the I signal side, an AGC circuit is formed from the variable gain amplifiers 8, 9, 10 and 11, and on the Q signal side, another AGC circuit is formed from the variable gain amplifiers 12, 13, 14 and 15. Where different variable gain amplifiers are provided for the I signal and the Q signal in this manner, even if the gains are controlled with the same gain controlling signal 22, a deviation in gain still occurs. For example, if total gain errors of the variable gain amplifier 8, 9, 10 and 11 and the variable gain amplifiers 12, 13, 14 and 15 are individually ±3 dB, then an amplitude deviation of ±6 dB in the maximum appears between the I signal and the Q signal. This corresponds to just twice or one half and makes a cause of quality deterioration at a decoding circuit in the following stage.
Meanwhile, an example of an amplitude deviation correction circuit of the type described is disclosed in Japanese Patent Laid-Open No. 94981/1995 (hereinafter referred to as document 1) and Japanese Patent Laid-Open No. 266498/1997 (hereinafter referred to as document 2). According to the technique disclosed in the document 1, the levels of an I signal and a Q signal are compared with a preset level, and the gain variation rates of amplifiers for amplifying the I signal and the Q signal are varied in response to the deviations. According to the technique disclosed in the document 2, the difference in amplification degree between two amplifiers for amplifying an I signal and a Q signal is minimized based on an integration value of the difference in output voltage between the I signal and the Q signal. A different example is disclosed also in Japanese Patent Laid-Open No. 22859/1998 (hereinafter referred to as document 3).